1. Field of the Invention
The invention relates generally to growth differentiation factor (GDF) receptors, and more specifically to GDF-8 (myostatin) receptors, to compositions that affect myostatin signal transduction in a cell, and to methods of using such compositions to modulate myostatin signal transduction in a cell.
2. Background Information
The amount of time, effort and money spent in the United States each year by individuals intent on losing weight is staggering. For many of these individuals, the goal is not merely to look better, but more importantly to avoid the inevitable medical problems associated with being overweight.
Greater than half of the adult population in the United States is considered to be overweight. Furthermore, twenty to thirty percent of adult men and thirty to forty percent of adult women in the United States are considered obese, with the highest rates occurring among the poor and minorities. Obesity, which is defined a being at least about twenty percent above the mean level of adiposity, has dramatically increased in prevalence over the past few decades and is becoming a major problem among the pediatric population. Twenty percent of all children are now considered overweight, a number that represents a doubling over the past five years.
Obesity and the medical problems directly attributable to it are a major cause of morbidity and mortality throughout the world. Obesity is a major risk factor for the development of various pathologic conditions, including atherosclerosis, hypertension, heart attack, type II diabetes, gallbladder disease, and certain cancers, and contributes to premature death. Heart disease is the leading cause of mortality in the United States, and type II diabetes afflicts over 16 million people in the United States and is one of the leading causes of death by disease.
More than eighty percent of type II diabetes occurs in obese persons. Although type II diabetes affects all races, it is particularly prevalent among Native Americans, African Americans and Hispanics. Significantly, type II diabetes, which used to occur almost exclusively in adults over age forty, now occurs in children, with reported cases having almost tripled over the last five years. Type II diabetes, also called non-insulin dependent diabetes, is characterized by reduced secretion of insulin in response to glucose and by resistance of the body to the action of insulin, even though insulin levels in the circulation generally are normal or elevated. Type II diabetes affects the function of a variety of different tissues and organs and can lead to vascular disease, renal failure, retinopathy and neuropathy.
In contrast to the medical problems associated with obesity, the severe weight loss that commonly occurs in patients with certain chronic diseases also presents a challenge to medical intervention. The molecular basis for this weight loss, referred to as cachexia, is not well understood. It is clear, however, that cachexia complicates management of such diseases and is associated with a poor prognosis for the patients. The effects of cachexia are evident in the wasting syndrome that occurs in cancer and AIDS patients.
Although great efforts have been made in attempting to elucidate the biological processes involved in regulating body weight, the results have provided more fanfare than actual value. For example, the discovery of leptin has been hailed as a breakthrough in understanding the molecular basis for fat accumulation in humans, and, with it, the promise of a cure for obesity. Studies in animals indicated that leptin is involved in transmitting internal signals regulating appetite, and suggested leptin could be useful for treating humans suffering from obesity. Progress in using leptin for treating obesity has been slow, however, and, thus far, leptin has not met initial expectations.
Treatment of the morbidly obese currently is limited to surgery to remove portions of the intestine, thereby reducing the amount of food (and calories) absorbed. For the moderately obese, the only “treatment” is eating a healthy diet and exercising regularly, a method that has proved modestly successful at best. Thus, a need exists to identify the biological factors involved in regulating body weight, including muscle development and fat accumulation, such that methods for treating disorders such as obesity and cachexia can be developed. The present invention satisfies this need and provides additional advantages.